How do you tell which gamecode belongs to which game?

If you activate the game from the Steam website, first open the dev tools (F12 on Chrome) and be sure to be on the "Network" tab.

You will be able to see the response from the server which includes the game's name and AppId.

For a step-by step instruction the first post in a thread on our Steam group outlines a few possibilities to get the answer:
https://steamcommunity.com/groups/freegamesfinders/discussions/2/2572002906853021998/

The browser-section copied here for your convenience:
1 - Go to https://store.steampowered.com/account/registerkey (make sure you're logged in with the account on which you want to activate the key!)
2 - Open the browser developer console with F12
3 - Submit your key in the product activation box on the Steam page (not the console)
4 - In the console, locate the tab titled as "Network"
5 - Click the "/account/ajaxregisterkey" entry
6 - Locate the "Response" tab
7 - Locate and expand the "line_items" section to reveal information about the Steam key

this will show your last activated games
https://store.steampowered.com/account/licenses/

p.s. if you're doing this just to see what a game is and not to activate them just know sometimes the key will activate. mainly if it or the one you have is part of a bundle but it can activate for other odd reasons.

I think one of the easiest way is sorting your Steam library by Recent, the top one must be the game that you just activate (or another latest played game if you play another game after activating your key).

Use the the steamdb extension in the browser, it shows the game directly.